Rap fans danced to country music, elderly white men high-fived with young African Americans and tears mixed with laughter as a varied lineup of A-list stars and an equally diverse crowd jammed the grounds of the Lincoln Memorial to celebrate the nation and its historic president-elect yesterday.

By some estimates, more than 400,000 people filled the western end of the Mall for the official start of a three-day jubilee of prayers, parades and parties. They endured long security lines and chilly weather for a two-hour salute to the man who will be America's first black president and to the nation that elected Barack Obama to the White House despite centuries of racial divisiveness.

"What a feeling this is! Good God, yes!" Mariela Jesse, a special education teacher from New York, shouted as Obama was introduced, her hands raised in the air.

Bruce Springsteen, Bono and Garth Brooks took the stage, but no one remotely touched the star power of one guest: Every time the Jumbotrons flashed a shot of the president-elect, a thunderous roar erupted from the farthest reaches of the crowd.

"We are all in this together," said actor Denzel Washington, adding that that is why the ceremony name was "three simple words: We Are One."

If the event served as a test run for what is expected to be a record crowd for tomorrow's inauguration, the lesson was simple: Arrive early. Traffic on some roads approached gridlock. But the day's biggest headaches were long delays at security checkpoints, and some people never made it in.

Today's events will center on service projects to mark Martin Luther King Jr. Day and are not expected to strain traffic and security. But officials and visitors alike are bracing for tomorrow's swearing-in ceremony and parade and the unprecedented crowds expected to come to witness the moment when Obama becomes the 44th president.

As Obama took the stage in front of the Lincoln Memorial, sharpshooters were visible on the roof. He told the crowd that he remains optimistic about the nation's future despite the challenges posed by war and economic crisis.

"And yet, as I stand here tonight, what gives me the greatest hope of all is not the stone and marble that surrounds us today, but what fills the spaces in between," he said. "It is you -- Americans of every race and region and station -- who came here because you believe in what this country can be and because you want to help us get there. . . .

"You proved once more that people who love this country can change it. And as I prepare to assume the presidency, yours are the voices I will take with me every day I walk into that Oval Office -- the voices of men and women who have different stories but hold common hopes. Who ask only for what was promised us as Americans: that we might make of our lives what we will and see our children climb higher than we did."

Obama, Vice President-elect Joseph R. Biden Jr. and their families sat to the side of the stage, behind a screen of protective glass. Obama and his wife, Michelle, bobbed their heads in time with the rhythm of the music.

The crowd was jubilant and orderly, but its sheer size had strained agents' screening effort.